In networks of neurons found in biological systems one of the most important elements has been found to be the interconnection between different neurons. This interconnection is referred to as a synapse. A neuron operates to integrate inputs, which enter the neuron from synaptic junctions located primarily on nerve structures known as dendrites, and to generate an output which is propagated on the neuron's axon. The output of the neuron is transmitted to other neurons by means of additional synapses between the axon and the dendrites on the other neurons. Networks of neurons, interconnected in this manner, provide the diverse functions of the central nervous system.
There are two types of synapses, excitatory and inhibitory. The excitatory synapses have the effect of enhancing the output of the target neuron. That is, electrical pulses entering through an excitatory synapse tend to promote the generation of electrical pulses in the neuron. Conversely, inhibitory synapses have the effect of suppressing the output of the neuron. Although the present understanding of the functioning of neurons is limited, it appears that the inhibitory synapses are fixed in strength, whereas the strength of the excitatory synapses is modifiable. That is, the effect of an excitatory synapse upon the neuron varies over time as a function of the input to the synapse.
An important factor in the operation of biological neurons is the chemical and electrochemical environment of the neurons. For example, it is known that the diffusion of ions, such as sodium and potassium, into and out of the nerve cell forms the underlying mechanism for generating the electrical potential of the nerve cell.
An artificial neural network is a system having arbitrary computational properties that is constructed from elements that exhibit neuron-like properties. The elements may be simulated in software or constructed from electrical or electronic components.
By example, European Application No. 0349007 discloses a semiconductor integrated circuit for constructing a neural network model and includes an inhibitory and an excitatory synapse circuit.
Hiraki et al. in U.S. Pat. No. 4,240,096 disclose a semiconductor device comprising a fluorine ion implantation region which is selectively formed in a semiconductor region and further activated. Fluorine ions are implanted in the selected portion of the semiconductor regions, followed by heat treatment. The implanted fluorine ions are provided to act as a carrier trap to extinguish carrier electrons.
In a publication by A. M. Hartstein et al., IBM Technical Disclosure Bulletin, Vol. 31, No. 7, December 1980, pages 254-257, entitled "Programmable Associative Memory Implemented in Silicon Technology" an associative memory network is described that includes the use of a single MOSFET device which interconnects other elements of the network and which functions as a threshold switch. To control threshold voltages of the MOSFET, the gate region may be ion implanted. A preferred technique is said to employ charge that is injected and trapped within individual gate oxides.
Other U.S. Patents include the following. U.S. Pat. No. 3,341,754, issued Sep. 12, 1967, entitled "Semiconductor Resistor Containing Interstitial and Substitutional Ions Formed by an Ion Implantation Method" to C. M. Kellett et al. discloses the implantation of boron or phosphorous into silicon during the fabrication of a resistor. U.S. Pat. No. 4,868,615, issued Sep. 19, 1989, entitled "Semiconductor Light Emitting Device Using Group I and Group VII Dopants" to Kamata discloses the implantation of compound semiconductors in the formation of light emitting devices.